think your attack dog analogy concedes more than you might realize
"we train our own beast" only works if someone actually holds the leash
with the state, the beast holds the leash, defines who counts as a threat, funds itself by force, judges its own agents, and calls its own violence lawful
Except the officials of the state are elected by the people. And if they abuse their power, they lose the next election and get put on trial by the next administration. It's happened more than once, in multiple countries. So it seems that they can self-regulate just fine.
Your flaw is separating yourself from the state and acting like the state is some alien entity, when in reality, it's your neighbors, your fellows, and people like you who fill the shoes of who gets into state jobs like police, firefighters, and even local state or city councils.
And unlike mafias or corporations, a state has obligations for the upkeep of society, and self-regulatory systems that ensure an abuse of state power will come with a steep punishment.
also, "humanity decided this" is just fake collective consent. Actual people are born under existing jurisdictions, they did not sign anything. Conquest, habit, fear, and lack of alternatives are not consent
Except people continue to agree to these systems. If they didn't, they'd have rebelled against it. Like how the poor of Rome helped the upstart Caesar overthrow the Roman Senate, for instance. Most people have judged that keeping the current system is preferable to doing something else.
My alternative is not "no enforcement", but no monopoly enforcement. Defense, arbitration, restitution, insurance, mutual aid, local autonomy, competitive service provision, and voluntary institutions instead of one compulsory apparatus claiming the right to rule everyone
you keep asking for a perfect substitute for the state while accepting endless imperfection from the state itself. There's nothing realist about it, it's just special pleading for your preferred monster
Except we already tried that. The Middle Ages had power be split up between many feudal and ecclesiastical magnates who had the power to check each other. The US under the Articles of Confederation had no single government, instead it delegated power to many local state authorities. Both systems failed and were replaced either by a centralized monarchy or a centralized federal democracy. Often with the support of the third estate. The third estate especially wanted a strong government that can keep the rich and powerful in line, so historically, they supported stronger governments. In the late Middle Ages and early Renaissance, they supported absolute monarchs against the nobles. A hundred years ago, they supported the Progressive movement against corporate abuse of workers and the environment.
And you also have no practical means of getting to your ideal utopia. Again, with the slavery analogy that you brought up, slavery was eradicated by two factors; slavery being an inefficient economic system, and roving bands of abolitionist marauders who were later joined by the federal government in the cause of abolishing slavery by force.
You have no practical means to get to your utopia, henceforth all your babbling is nothing but talk.